1. Field of the Invention
The present invention is related to image sensing integrated circuits, and more particularly, to a method for chip scale packaging using an adhesive matrix.
2. Background Information
Currently, prior art chip scale packaging for optical image sensor devices require that a wafer be completely coated with an optically clear adhesive and subsequently covered with a transparent covering material. Typically, the optically clear adhesive can be an epoxy and the transparent material is glass.
However, nearly all image sensors require the application of micro lenses to increase the optical fill factor to meet performance objectives. As seen in FIG. 1, a typical chip scale package for an imaging sensor is shown. The packaging technique includes a wafer 101 having the image sensor chips formed thereon. Micro lenses 103 are formed atop of the wafer 101 to cover each pixel (picture element) individually. After pads have been extended into the scribe lines, an adhesive layer 105 is then applied, typically using some type of homogenous spin coating technique. Finally, a cover glass 107 is applied to the adhesive. The adhesive 105 serves to secure the cover glass 107 to the wafer 101. Typically, the adhesive 105 is an epoxy.
Next, the backside of the wafer 101 is ground down to make the wafer thinner. Next a wafer sawing operation is used to expose the extended image sensor electrical leads though the back side of the thinned wafer. The exposed leads are then extended to form electrical contacts with traditional deposition and etching processes. Finally, a die cutting machine is used to cut the wafer 101 into individual image sensors.
However, this prior art method of chip scale packaging is problematical for image sensors because the refractive index of the micro lens material is very similar to the adhesive 105. Thus, the adhesive 105 tends to counteract the optical effect of the micro lenses 103.